1. Field of the Invention
The present invention relates to rockbolts made of steel pipes and, more particularly, to rockbolts to be expansively embedded in a bedrock or ground for reinforcement.
2. Description of Related Art
A bedrock or ground with fear of spring water or sudden fall is conventionally reinforced by embedding deformed bar type rockbolts therein. Recently, expansive rockbolts, which are hammered and expanded in a bedrock or ground, have been employed instead of the deformed bar type rockbolts.
A conventional expansive rockbolt is made of a deformed steel pipe, which has an expansive groove extending along its axial direction and a sealed top end, as disclosed in JP 2-5238 B. The expansive rockbolt is placed in a hole of a bedrock or ground, after a sleeve for introduction of a pressurized fluid is attached to a rear end of the rockbolt. Thereafter, a pressurized fluid is forcibly injected into the rockbolt through an opening formed at a side of the sleeve, so that the deformed steel pipe is expanded and pressed onto an inner wall of the hole. As a result, the bedrock or ground is reinforced by fixation of the expanded rockbolt. Expansive rockbolts, which have joints attached to sleeves for supply of a pressurized fluid, are also disclosed by JP 2003-206698 A and JP 2004-019181 A.
An expansive rockbolt provided with a joint for introduction of a pressurized fluid has a main body 1, to which a sleeve 2 for introduction of a pressurized fluid is attached at its rear end, as shown in FIG. 1. An opening 3 for injection of a pressurized fluid is formed at a side of the sleeve 2, and both sides of the opening 3 are shaped to a cylindrical part 4 for sealing with packing. A large diameter flare 5 is formed at an end of the cylindrical part 4 for enlargement of a surface area in contact with a bearing plate 6. Formation of the cylindrical part 4 and the flare 5 unavoidably put restrictions on a length of the sleeve 2, but the sleeve 2 can not be shorter than a predetermined length. As a result, the sleeve 2 projects from the bearing plate 6 higher than a conventional deformed bar type rockbolt, when the rockbolt main body 1 is set in a hole of a bedrock or ground.
By the way, in a construction site such as a tunnel, a bedrock or ground is drilled through a sprayed concrete layer for formation of a rockbolt-setting hole, a rockbolt is set in the hole, and then the rockbolt is hydraulically expanded for reinforcement of the bedrock or ground. Thereafter, the sprayed concrete layer is covered with a waterproof sheet 7, and lining concrete 8 is placed thereon, as shown in FIG. 2.
During placing the lining concrete 8, the waterproof sheet 7 often tears due to the projected sleeve 2. The lining concrete 8 becomes thinner at a part corresponding to the projected sleeve 2. The waterproof sheet 7 is prevented from tearing by attachment of a cap to the projected sleeve 2 in prior to covering with the waterproof sheet 7. However, attachment of the cap not only requires additional labor and time but also makes the lining concrete 8 thinner, resulting in poor strength. Moreover, if the lining concrete 8 is dislocated from the sprayed concrete layer due to thermal expansion and shrinkage, the lining concrete 8 is sometimes cracked 9 at a position near a top of the projected sleeve 2.